(1) Field of the Invention
The present invention is directed to a cascode radio frequency amplifier. The cascode amplifier of the present invention provides equal voltage and power sharing of cascode amplifier stages. Each amplifier stage provides unity gain and is a clone of a first control amplifier stage. The device operates over a frequency range from the low frequency band to the microwave band.
(2) Description of Related Art
Traditional solid-state microwave amplifiers employ a system of amplifiers connected in cascade. An example of a typical cascade power amplifier chain will include, for example, five amplifiers, each of which has a power gain of 6 dB, a power splitter having a ratio of 1:4, and 3 dB couplers. The power splitter is connected between the first amplifier and four of the other amplifiers. The 3 dB couplers is connected to the output of the four serial amplifiers. At an input power of 24 dBm (250 milliwatts) the first amplifier would provide 30 dBm to the input of the power splitter. The output of the power splitter would then provide 24 dBm to each of the four amplifiers connected thereto. The 30 dBm (1 watt) output from each of the amplifiers is then supplied to the 3 dB couplers. Two of the outputs from two of the amplifiers are connected to a first coupler and two of the outputs from two of the remaining amplifiers are connected to a second coupler. The output from the couplers are combined in an output coupler which provides a final output of 36 dBm (4 watts). The output power from each of the circuits, however, does not take into account matching, combining, and power splitting losses which would be on the order of one decibel. The efficiency of each amplifier is between 40 and 50% and the total efficiency of a cascade amplifier of this type would be approximately 45% which results from rectifier and series regulator losses. Each 1 watt amplifier stage, if the regulated drain D/C power supply efficiency is on the order of 70%, has an overall efficiency of 31.5%. The total input power required for a circuit including five 1 watt amplifier stages is 15.9 watts. This yields a total efficiency for a 4 watt radio frequency output power of 25%. Although cascode RF amplifiers have been tried, due to the complex biasing networks required for series operation of the transistors, they have not been successful beyond two stages. This is because each stage must be referenced to ground, and the more stages added, the more complicated the circuitry becomes. Therefore, radio frequency amplifiers have been limited to employing cascade amplifier chains in order to achieve power amplification.